It is now known that intramyocellular triglycerides (imcTG) content in skeletal muscle of obese adults is increased and this abnormality is associated with impaired glucose metabolism in the muscle. However, the pathways responsible for the increase and the link between the increased imcTG and insulin resistance have not been studied in detail. The objective of this application is to determine the factors and pathways that are responsible for the imcTG accumulation, and to determine whether the oxidation of imcTG fatty acids is also increased and, if so, whether it directly affects glucose metabolism. It is hypothesized that elevated plasma insulin and fatty acid levels, as commonly seen in human obesity, independently stimulate imcTG synthesis and synthesis is the primary pathway leading to the increased imcTG accumulation; and that a larger imcTG pool leads to accelerated imcTG oxidation thereby interfering with muscle glucose metabolism. To test the hypotheses, three specific aims will be pursued to answer following questions: 1) Is insulin an anabolic hormone stimulating imcTG synthesis? 2) Does elevated plasma fatty acid concentration increase imcTG synthesis by providing abundant precursors? 3) Is a larger imcTG pool associated with accelerated oxidation of imcTG fatty acids, and if so, how this affects muscle glucose metabolism? A new one-pool model will be applied to determine the rates of imcTG synthesis, turnover and oxidation directly (muscle biopsy) at controlled insulin and fatty acid levels in rats made obese by high fat feeding. The oxidation of imcTG fatty acids and muscle glucose uptake, glycolysis and glycogen synthesis will be determined using multiple tracers to determine the effect of imcTG oxidation on glucose metabolism. Stable isotopic tracers (13C) and mass spectrometry (GC/MS and isotope ratio MS) will be used to quantitate the kinetics. These studies are designed to answer the questions whether an enlarged imcTG is a chemical entity that imposes a negative effect on glucose metabolism, and whether plasma insulin and fatty acids are responsible for the increased imcTG, and if so, how. Thus, the proposed research will improve the understanding of the mechanism of insulin resistance and imcTG abnormalities in the obese rat that will benefit investigation of human obesity.